Captain America note Steve Rogers (Captain America), Sergeant James Buchanan "Bucky" Barnes, Colonel Chester Phillips, The Howling Commandos, Sam Wilson (The Falcon), Agent Margaret "Peggy" Carter, Dr. Abraham Erskine, Howard Stark, Brock Rumlow, Agent Sharon Carter (Agent 13), Secretary of Defense Alexander Pierce, HYDRA, Dr. Johann Schmidt (Red Skull), Dr. Arnim Zola, Heinz Kruger, The Winter Soldier, Baron Wolfgang von Strucker, Georges Batroc
The Incredible Hulknote Dr. Robert Bruce Banner (The Hulk), Dr. Elizabeth "Betty" Ross, General Thaddeus "Thunderbolt" Ross, Captain Emil Blonsky (The Abomination), Dr. Samuel Sterns
Iron Mannote Anthony "Tony" Stark (Iron Man), Virginia "Pepper" Potts, Obadiah Stane (Iron Monger), Harold "Happy" Hogan, J.A.R.V.I.S., Howard Stark, James "Rhodey" Rhodes (War Machine/Iron Patriot), Senator Stern, President Matthew Ellis, Vice President Rodriguez, Ivan Vanko (Whiplash), Justin Hammer, Aldrich Killian, Maya Hansen, Eric Savin, Ellen Brandt, Raza, Abu Bakaar, The Mandarin, Christine Everhart, Dr. Ho Yinsen, Harley Keener
Thornote Thor Odinson ("Donald Blake"), Loki Odinson, Allfather Odin Borson, Queen Frigga, Lady Sif and the Warriors Three, Heimdall, The Destroyer, Bor Burison, Dr. Jane Foster, Dr. Erik Selvig, Darcy Lewis, Ian, King Laufey, Malekith, Algrim (Kurse)
Guardians of the Galaxynote Peter Jason Quill ("Star-Lord"), Gamora, Drax, Groot, Rocket, Ronan, Nebula, Korath, Irani Rael, Rhomann Dey, Garthan Saal, Yondu, Kraglin, The Collector, Carina, The Broker, Howard the Duck
S.H.I.E.L.D. (the organization)note Director Nicholas J. "Nick" Fury, Agent Maria Hill, Secretary of Defense Alexander Pierce, The World Security Council, Agent Phil Coulson, Agent Natasha Romanoff (Black Widow), Agent Clint Barton (Hawkeye), Agent Jasper Sitwell, Agent Felix Blake, Benjamin "Benny" Pollack, Claire Wise
Another Dimension: Or at least a "new universe", if Erik Selvig's comment in The Avengers is anything to go by.
Arm Cannon: The cannons that some of the Chitauri have (including the eponymous Item 47) seem to be fit around their arms to fire one-handed. They, however, do not seem to actually be the arm, since an arm falls out when Captain America chops it off of one of them.
Awesome Personnel Carrier: Their main transport is the Leviathan, a gigantic flying serpent-like creature that carries hundreds of soldiers and can fly through skyscrapers. It takes huge amounts of explosive or electrical damage to put one down. Or one Hulk punch and a several Stark missiles.
Or blow one up from inside.
Bayonet Ya: Their infantry sport them on their rifles.
Blood Knight: They grow restless when denied war. The tactical problems with this are summed up in a deleted scene:
Expy: Of Darkseid's Parademons. Which is fitting, since their leader, Thanos, was originally designed as a Darkseid expy.
Frickin' Laser Beams: Two varieties. One is fired from a thin spear/rifle used by Chitauri foot soldiers. The other is a powerful Arm Cannon that's mounted on Chitauri commanders and units riding in hovercraft. Both fire blue colored beams.
Humanoid Aliens: Barring the freaky double-thumbs and evil-looking skull-faces.
An alien who acts as the liaison between the master of the Chitauri and Loki.
Back for the Dead: Appears in Guardians of the Galaxy in a couple of scenes, acting as a middleman between Ronan and Thanos. When Ronan goes to see Thanos in person, he grows annoyed with the Other constantly arguing with him, and kills him.
Black Cloak: He wears a black robe that covers the top half of his face.
The Dragon: Appears at first to be working for Loki, but is quickly revealed to be the intermediary for the even more powerful Thanos. After his untimely death, it appears the Titan's adopted daughter Nebula will take his place instead.
Easily Conquered World: Believed Earth would be one of these, and that the entire Earth would surrender the minute the Chitauri landed.
Humanoid Aliens: He has the same general shape as a human (and of course is played by a human actor), but the details are very different. For starters, he has two thumbs on each hand. It's not really clear if he's of the same race as the Chitauri.
In the Hood: His hood helps to further obscure his face.
Leader of the Chitauri, who lent Loki his forces in exchange for Loki promising him the Tesseract in The Avengers. He later promises Ronan the Accuser that he will destroy the homeworld of the Nova Corps in exchange for the Orb in Guardians of the Galaxy.
0% Approval Rating: The guy has ChronicGetting-Backstabbed Disorder. Gamora and Nebula both hate him and abandon him the moment they see their chance, and the instant Ronan knows he can get what he wants without Thanos, he straight-out tells him that he (Ronan) is going to kill him (Thanos). The only characters who actually serve him without thoughts of betrayal are Loki (who's insane), and the Other (who's a Flat Character). Really, Thanos's biggest weakness is that the instant he turns his back on anyone, they bring out the knife.
Abusive Parent: He "adopted" both Nebula and Gamora after killing their respective families. He then surgically modified both with cybernetics to become his personal enforcers. Gamora calls her time with Thanos worse than any nightmare, and Nebula states that Thanos turned her into a monster.
Makes violent, inventive threats to both Loki (through the Other) and Ronan (in person).
Doesn't react at all when Ronan kills the Other in front of him.
Everyone who works for him in Guardians of the Galaxy takes the earliest possible opportunity to betray him.
Bigger Bad: While not directly involved in any of the early films he appears in, he's the galactic Evil Overlord driving events from behind the scenes. All in order to gather the Infinity Stones and take over the Universe.
In The Avengers, his mouthpiece tells Loki to get Thanos the Tesseract as promised, or suffer unimaginable pain.
He's the master of the Chitauri, who promised Loki Earth in exchange for the Tesseract in The Avengers.
In Guardians of the Galaxy, he commands Ronan the Accuser to find the Orb in exchange for his help against Xandar, putting Ronan into conflict with the main characters. However, once Ronan turns against him, Thanos is no longer involved with Ronan's actions. The film implies that Thanos has been backing Ronan for some time; after Ronan is killed, Drax considers Thanos the man truly responsible for his family's deaths.
World's Strongest Man: Ronan's second in command literally calls him "the strongest being in the universe". Even the most powerful of "superhuman" races like the Asgardians and Kree are afraid to cross him.
A pioneer of the comic industry, and the creator or co-creator of most of the characters so far featured in not only the Marvel Cinematic Universe, but also the X-Men, Spider-Man and other non-Disney film and franchises. Much like Alfred Hitchcock, it has become a tradition for Stan Lee to make cameo appearances in most movies or TV series based on Marvel Comics, MCU and otherwise.
As Himself: Except perhaps in Iron Man 1 and 2, where he may or may not have been Hugh Hefner and Larry King. Definitely not himself in Captain America, since he would been about 60 years younger at that point in time. Also not himself in Guardians of the Galaxy unless he's mastered in interstellar space travel.
Creator Cameo: With the exception of his appearance in Captain America. He did not create that character but was responsible for bringing him into the Silver Age, as well as creating Cap's now-iconic shield throwing.
He appears in Guardians of the Galaxy despite only having a hand in creating Groot. Even then, Stan's original characterization of Groot was incredibly different from what is seen in the modern comics and the film.
The Mandarin is the mysterious leader of the Ten Rings terrorist organization that menaced Tony in the first Iron Man film. Though Aldrich Killian's scheme to use the Mandarin's image made it seem that he was a mere fabrication, the Marvel short All Hail the King reveals that he is indeed real. Jackson Norris claims he was ordered by the Mandarin to break Trevor Slattery, the fake Mandarin, out of prison, evidently so the Diabolical Mastermind can punish his impostor personally.
Bigger Bad: He's behind the Ten Rings, but hasn't yet stepped into the spotlight with any evil plans.
The Ghost: So far, he's only been referenced by name in Iron Man 3 and All Hail the King, without appearing personally.
Meaningful Name: The word "Mandarin" comes from an ancient Chinese name for an "advisor to the King". Indeed, he's said to have advised powerful figures throughout human history.
Walking Spoiler: Revealing his existence gives away The Reveal from Iron Man 3 that the Mandarin appearing in that film is nothing but a Red Herring for Aldrich Killian, not to mention that Killian was lying about being the Mandarin all along.
Warrior Prince: He's described as a warrior king who's "inspired generations of men".
We Wait: Jackson Norris notes that his organization has been dormant for some time.